Re: [Qemu-devel] [RFC][PATCH 23/45] qemu-kvm: Rework MSI-X mask notifier to generic MSI config notifiers

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 17, 2011 at 01:45:04PM +0200, Jan Kiszka wrote:
> On 2011-10-17 13:40, Michael S. Tsirkin wrote:
> > On Mon, Oct 17, 2011 at 11:27:57AM +0200, Jan Kiszka wrote:
> >> MSI config notifiers are supposed to be triggered on every relevant
> >> configuration change of MSI vectors or if MSI is enabled/disabled.
> >>
> >> Two notifiers are established, one for vector changes and one for general
> >> enabling. The former notifier additionally passes the currently active
> >> MSI message.
> >> This will allow to update potential in-kernel IRQ routes on
> >> changes. The latter notifier is optional and will only be used by a
> >> subset of clients.
> >>
> >> These notifiers are currently only available for MSI-X but will be
> >> extended to legacy MSI as well.
> >>
> >> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> > 
> > Passing message, always, does not seem to make sense: message is only
> > valid if it is unmasked.
> 
> If we go from unmasked to masked, the consumer could just ignore the
> message.

Why don't we let the consumer get the message if it needs it?

> > Further, IIRC the spec requires any changes to be done while
> > message is masked. So mask notifier makes more sense to me:
> > it does the same thing using one notifier that you do
> > using two notifiers.
> 
> That's in fact a possible optimization (only invoke the callback on mask
> transitions).

Further, it is one that is already implemented.
So I would prefer not to add work by removing it :)

> Not sure if that applies to MSI as well, probably not.

Probably not. However, if per vector masking is
supported, and while vector is masked, the address/
data values might not make any sense.

So I think even msi users needs to know about masked state.

> To
> have common types, I would prefer to stay with vector config notifiers
> as name then.
> 
> Jan

So we pass in nonsense values and ask all users to know about MSIX rules.
Ugh.

I do realize msi might change the vector without masking.
We can either artificially call mask before value change
and unmask after, or use 3 notifiers: mask,unmask,config.
Add a comment that config is invoked when configuration
for an unmasked vector is changed, and that
it can only happen for msi, not msix.

This also removes the need to handle enable/disable specially:
you simply pretend all vectors are masked.


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